Dumnonii

The Dumnonii were Britons who inhabited Devon (which came from their name) and Cornwall in southwestern England.

History
Names for the goddess Domnu, goddess of the deep, the Dumnonii inhabited soutwestern England on the Cornish Peninsula and parts of Somerset and Devon. The Dumnonii were related by blood to Iceni, Demetae, Brigantes, Caledonians, and Ebdani, and they became allies of the Iceni in 272 BC. They were enemies with the Namnetes of Gaul across the sea, and convinced the Iceni to declare war to help them (the Iceni made peace many times, as it was virtually a cold war). The Dumnonii were an expansionist and devious people, but war with the Iceni was far from their thoughts.